A bandsaw is a cutting apparatus that turns or trains an endless loop of a cutting element having a serrated or toothed edge (hereinafter sawblade) around two spaced-apart wheels. It is often used for cutting wood.
More particularly with reference to FIG. 1, a typical bandsaw 10 has a drive wheel D1 which drives a driven wheel D2 by turning a sawblade SB which loops around the two wheels. The sawblade is typically formed of steel. Bandsaws are provided in sizes that are small enough to allow for use on a table-top, and range up to the very large sizes used in sawmills for cutting logs, where the distance between the rotational centers C1 and C2 of the wheels is typically 10 feet or more.
The sawblade slides across a pair of “guides” G1 and G2 that push it outwardly, away from centerline CL of the wheel centers. This causes the sawblade to make a slight positive angle θ relative to the centerline CL as it leaves the driven wheel D2 and approaches the upper guide G2 (or alternatively, depending on the direction of rotation of the wheels, as it approaches the driven wheel after having left the upper guide), and as it approaches the driving wheel D1 after having left the lower guide G1 (or alternatively, depending on the direction of rotation of the wheels, as it leaves the driving wheel and approaches the lower guide).
A log “L” to be cut lengthwise with the sawblade SB is fed between the guides G1 and G2, in a direction perpendicular to the plane of the Figure, typically for producing “cants” and “flitches” as indicated in FIG. 2.
It is desired to maintain a substantially constant (tensile) stress on the sawblade, which decreases when the sawblade lengthens. To compensate for this lengthening, or “strain,” the sawblade, the position of the driven wheel D2 is adjusted along the axis indicated as “A” by a strain adjustment mechanism indicated generally as “SM.”
FIG. 3 shows an example of such a mechanism employing a typical elastomeric bellows “BL.” It may be noted that, due to the fulcrum and lever arrangement shown for mounting the driven wheel D2, the axis “A” is actually a circular arc, but for small rotations of the wheel center C2 about the pivot point P1 this are is reasonably approximated by a straight line.
The bellows is served by a pressurized line (air or hydraulic fluid) which pressurizes it to a degree determined by a human operator who initially sets the pressure in the line. The position of the driven wheel is a consequence of a balance between the expansion force of the bellows and the resistance of the sawblade, which provides for a position adjustment whereby the driven wheel moving farther apart from the drive wheel as the sawblade becomes increasingly strained. The expansion force provided by the bellows remains constant, independent of the strain, so long as the pressure setting remains unchanged, resulting in a constant stress on the sawblade at all times.
U.S. Patent Publication No. 2015/0158097 describes a problem known in the art as instability (referred to as “an oscillation . . . called ‘snaking’ . . . ”) of the sawblade, and proposes as a solution to sense changes in the forces detected by force sensors at the guides, particularly the guide that is downstream of the log, in terms of the direction of movement of the sawblade, i.e., the guide G1 if the sawblade circulates counterclockwise, or the guide G2 if the sawblade circulates clockwise, in FIG. 1. These changes are described as being precedent to instability of the sawblade, and are used to control the speed of feeding the workpiece to the bandsaw, hereinafter “feed-speed.”